I see that LEGO rubber bands/gaskets are often associated with a number preceded by an 'x,' such as 'x71' for the small/18 mm bands. A few bands also have a 'K' prefix, such as 'K4' for the large/30 mm units. This latter item is also known as and 'x89' rubber band.
I'd like to know what these prefixes mean and where they came from. Likewise, I'm curious what the actual numbers refer to.